When working road service I always broke my estimates down by parts and labor, not including sales tax or shipping cost and I let my customers know this. I always gave myself a little lee way on labor time just in case I ran into those irritating things like stripped screws, etc., If everything went right with no gremlins popping up then I would come in under the estimate. When it was time for the customer to sign my ticket I made a point of highlighting the labor and letting him/her know that I came in under the estimated cost of repair. They were usually very appreciative and I got repeat calls from them. As a matter of fact, quite a few (including the one I work for now) did away with the need for estimates altogether after awhile and just gave me a max/repair and no need for approval under the max. If you said it was going to take you from 8am-12pm to do the repair but left at 10:30am but still billed them for the full estimate until 12pm, aren't you setting yourself up for a possible lawsuit?